1. Field of the Invention
The present invention relates to a voltage application time period measuring circuit including a semiconductor device, for measuring a voltage application time period of a voltage applied from a commercial power supply to an electronic device, and a power supply apparatus including the voltage application time period measuring circuit.
2. Description of the Related Art
A conventional voltage application time period measuring circuit is described. FIG. 17 illustrates the conventional voltage application time period measuring circuit.
When an electronic device (not shown) is connected to a commercial power supply (not shown), an alternating current voltage from the commercial power supply is input to a voltage application time period measuring circuit 100. Then, a zero-cross point detecting circuit 101 detects a zero-cross point of the alternating current voltage and generates a pulse signal. A counter 102 counts the pulse signal. A reference value setting circuit 103 has a predetermined reference value corresponding to a product life set therein. A comparator 104 compares the reference value from the reference value setting circuit 103 with a count value of the counter 102. When the count value is equal to or larger than the reference value, an output voltage of the comparator 104 is inverted, and hence it is determined that the life of the electronic device reaches the product life.
When the electronic device is disconnected from the commercial power supply, the counter value of the counter 102 is held by a backup battery 200 (see, for example, Japanese Patent No. 3264473).
However, the technology disclosed in Japanese Patent No. 3264473 requires the backup battery 200, and hence a power supply apparatus cost is increased correspondingly.